1. Field of the Invention
The present invention relates to a system for the transmission of signals between a signal reception assembly lowered into a well and a central control and recording unit disposed outside the well. The transmission system of the invention is more particularly suitable for a signal reception assembly comprising an acquisition device adapted for collecting signals received by sensors disposed in a well, digitizing them and translating them into coded form for transmission thereof. 2. Description of the Prior Art
Measures in bore holes are made using sensors generally contained in at least one probe lowered from the surface at the end of an electric supporting cable. The probe includes one or more arms actuated by hydraulic jacks associated with a system delivering a pressurized fluid. Opening of these arms, which anchor themselves in the wall of the well and thus immobilize the probe at a depth where measurements are to be made, is controlled from the surface installation using different conducting lines of the support cable. Either an electric current for feeding an electric motor and thus compressing the hydraulic fluid by operation of the jacks, or an electric signal for actuating control solenoid valves are transmitted from the surface installation. A probe may also be used for example having anchorage arms actuated by springs held under tension by bolts. Hydraulic means, controlled by opening solenoid valves, cause unlocking and opening of the arms. These solenoid valves are controlled by transmitting, from the surface installation, electric signals over a conducting line of the support cable.
Such, probes are described for example in U.S. Pat. Nos. 4,428,422 or 4,616,703.
The different sensors disposed in the well for making measurements may be disposed in a single probe or else in an assembly of probes comprising a main probe under which a plurality of satellite probes are suspended at different depths. Such a probe assembly is described for example in French patent No. 2 564 599.
Thus an assembly of sensors is obtained spaced apart over a depth which may reach several tens even several hundreds of meters, which makes it possible for example to carry out seismic prospection operations; with a seismic source tripped at the surface or in another well, for example, the acoustic signals reflected back by the discontinuities of the subsoil are received by different sensors, and these signals are used so as to obtain seismic sections of the subsoil.
Transmission of the different signals picked up to a surface seismic recording unit may take place in analog form when the number of sensors is relatively small. But when the reception assembly comprises a higher number (more than six for example taking into account the type of cable used in practice) digitized and coded transmission is generally used by adapting the transfer rate to the capacities of the transmission cable supporting the probe.
From the U.S. Pat. No. 4,355,310 a digitized signal transmission system is known making it possible to connect well probes containing sensors of very different kinds: acoustic, nuclear, etc, to a control unit disposed on the surface. All the data connected by the different sensors are digitized and centralized by a control member disposed in the vicinity. The connection between the central control and recording device disposed on the surface, is provided by means of modems connected to transmission lines of the cable. The speed of transmitting the coded signals is adapted to the possibilities of the cable used. It is generally of the order of a few tens of kilobits/sec and depends on the type of coding used. It is generally the transmission rate made possible by the transmission cables used for connecting the well probes to a surface installation.
One of the cable patterns most generally used includes seven parallel lines. Six lines are distributed evenly about a central line. A metal sheath in the form of a braid is disposed at the periphery of the cable so as to give it mechanical strength and withstand tractive forces. The transmission of coded digitized signals is achieved using certain lines at the periphery of the central line.
The prospection methods developed at the present time for increasing the speed of seismic exploration of underground profiles lead to using complex reception devices having a large number of seismic receivers distributed very often in several probes stepped at different depths, so as to multiply the number of positions where waves are picked up simultaneously for recording thereof. The volume of data to be transmitted increases considerably.
In some cases where the acquisition time of the signals is short with respect to the time interval between two transmission-reception cycles, the acquired data may be stored in a storage means of one of the probes and their transmission to a surface recording system is delayed. But very often the data acquisition is carried out practically continuously.
This is true particularly for certain vibroseismic applications where the interruption time intervals between the successive acquisition periods are too short to contemplate delayed transmission. It is therefore imperative for the data to be transmitted in real time and so for the flow rate of the transmission cables to be considerably increased;
The system of the invention avoids the above drawbacks.